1. Field of the Invention
The present invention relates to a positive electrode for a non-aqueous electrolyte cell and a non-aqueous electrolyte cell and in particular, to improvement of a spinel-type lithium composite manganese oxide as an active material.
2. Description of Related Arts
Recently, high-performance, small-size, portable electronic devices have been developed and a secondary cell of a high energy density is required as a drive source for these devices. As a secondary cell used for these electronic devices, there can be exemplified a nickel-cadmium secondary cell, a lead accumulator, nickel-hydrogen secondary cell, lithium-ion secondary cell, and the like. Among them, the lithium-ion secondary cell has a high cell voltage, a high energy density, small self-discharge, and no memory effect.
Currently, in the lithium-ion cell, it is usual that lithium cobaltate (LixCoO2, 0≦x≦1) having a layered structure is used for the positive electrode and carbon having a layered structure is used for the negative electrode. The lithium cobaltate exhibits a voltage characteristic of 4 V or above and can retain a comparatively stable structure for the lithium-ion doping and dedoping. However, cobalt is limited as resources and expensive. Accordingly, it is desired to use a positive electrode material not containing cobalt.
As a candidate of the positive electrode material not containing cobalt, there can be exemplified lithium nickelate having layered structure (LixNiO2, 0≦x≦1) and lithium manganate (LiMn2O4) having spinel structure. Especially, the lithium manganate is cheap and has a high safety, attracting attention as a positive electrode material of the next generation.
However, the lithium manganate has a problem that during a storage at a high temperature and a cycle, the manganese is dissolved into the electrolytic solution, deteriorating the cell characteristic. For practical use, it is necessary to improve the cell characteristic by suppressing the dissolving of manganese.
In order to prevent dissolving of manganese into the electrolytic solution, for example, as is disclosed in Japanese Patent Publication 9-147859, it is effective to reduce the specific surface of the positive electrode active material so as to reduce the contact area with the electrolytic solution. It is true that reduction of the specific surface can reduce the dissolving of the manganese but the reduction of the specific surface results in increase of the particle diameter, deteriorating the reaction at lithium doping and dedoping, which in turn deteriorates charge-discharge characteristic at a high load. The reason of the deterioration of the reaction is considered to be that when the particle diameter is increased, the lithium-ion dispersion becomes slower. In order to increase the speed of the lithium dispersion, the crystal particle diameter should be small. Thus, the particle diameter control alone cannot achieve both of the cycle characteristic and the high load characteristic.